1. Field of the Invention
The present invention relates to wastewater treatment systems and more particularly, to such systems and methods using fluidized bed reactors.
2. Description of Related Art
Biological fluidized bed reactors have been widely used to remove dissolved and suspended organic matter from wastewater. A fluidized reactor uses bacterial biofilms attached to media to biologically convert wastewater constituents to desirable end products. Examples are the conversion of organic carbon to carbon dioxide and the conversion of organic nitrogen into ammonia or nitrate to atmospheric nitrogen. Within a fluidized reactor circulation of water or sparging by air causes media particles to move and circulate. Media particles then abrade each other, resulting in the scouring of excess biofilm from the surface of media particles. The excess biofilm may continue to circulate within or pass out of the reactor, depending on the design of the reactor. These biofilm reactors may be aerobic, anoxic, or anaerobic as called for in a particular application.
The fluidized bed reactors use an attached biofilm fluidized bed process that is based on the concept that large biomass concentrations can be achieved on a large surface area by dense biofilm attachment to an inert small particle size carrier. A large surface area is created by small inert particles in a bed, fluidized by upward flow or air sparging. The intense mixing occurring in the bed minimizes diffusion limitations and eliminates clogging and short-circuiting problems. These reactors accumulate large amounts of active biomass, and can achieve very long cell detention times. In some designs, the bed is fluidized by the upward flow through the column of untreated wastewater mixed with recirculated effluent. The upward flow necessary to achieve bed fluidization is distributed by means of various nozzles or small-diameter pipes placed at the bottom of the column. Biofilm develops on the inert media, and the physical attachment of anaerobic bacteria to the media surface prevents biomass washout. The high fluid shear force resistance of biofilms allows these reactors to be operated at upflow velocities that would otherwise wash out unattached biomass.
Under some conditions the turbulent flow exerts sufficient shear to prevent the development of thick biofilms on the media, which limit mass transfer. The high surface-to-volume ratio of the bulk of the bed inert media (300 to 2500 ft2/ft3) creates a vast area for the development of microbial biofilm. Approximately 95 percent of the active biomass in a well-operated fluidized bed reactor is attached growth. This fact enables the development of dense but thin biofilms that lead to high concentrations of attached biomass in the bed.
Despite the great advantages that this process offers in terms of high organic loading rates, short hydraulic retention times, and low excess solids generation, the use of these reactors has not been extensive owing to various design limitations.
Previously submitted patent applications, U.S. Pat. No. 2003/0230529 and U.S. Pat. No. 2004/0000517, commonly owned with the present invention, have introduced the concept of nitrification and denitrification mediated by tidal flow. The disclosures of these applications are hereby incorporated hereinto by reference.